Ultrasonic diagnostic apparatus and method

ABSTRACT

An ultrasonic diagnostic system according to the present invention has an ultrasonic transducer mounted on a catheter, an instantaneous distortion value calculator which calculates an instantaneous distortion value at the location to be evaluated in the blood vessel based on received signal representative of received reflection by the ultrasonic transducer, an analyzer which analyzes a state of the location to be evaluated using time-series data of instantaneous distortion values which are calculated at a plurality of times in a chronological sequence by the distortion value calculator, and a display which displays an analyzed result produced by the analyzer.

This application is based on Japanese Patent Application No.2003-129173filed on May 7, 2003, the contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to an ultrasonic diagnostic apparatus andmethod for analyzing an intravascular state, and more particularly to anultrasonic diagnostic apparatus and method for diagnosing the propertyand state of an intravascular sclerotic nidus called “plaque” accordingto an intravascular echoing process.

Signs and symptoms that result in an observation of acute ischemia, suchas unstable angina and acute myocardial infarction, are called acutecoronary syndrome. Heretofore, it has been considered that the acutecoronary syndrome is caused by a constriction or obstruction in a bloodvessel that is produced by a gradual accumulation of plaque (scleroticnidus) within the blood vessel over many years.

In recent years, however, a certain type of plaque that is formed in ablood vessel is thought to be responsible for the development of acutecoronary syndrome. Generally, a plaque comprises a soft atheromatouslipid component and a relatively hard fibrous capsule (fibrouscomponent). A plaque having a higher lipid content is a vulnerableplaque that can easily be broken by even a small stimulus. A plaquehaving a lower lipid content is a plaque that is less liable to bebroken. The vulnerable plaque with the higher lipid content is softerand more deformable than the plaque with the lower lipid content. Theformer plaque is called a soft plaque, whereas the latter plaque iscalled a hard plaque. When the vulnerable soft plaque is broken, itproduces a thrombus, causing a constriction or obstruction in the bloodvessel which tends to bring about the acute coronary syndrome.

For preventing and appropriately treating the acute coronary syndrome,it is necessary to establish a technique to evaluate the property andstate of plaques. Particularly, there is a need for adequatelydetermining whether a plaque formed in a blood vessel is a vulnerablesoft plaque or a hard plaque.

For determining the property and state of plaques, it is better to usean intravascular echoing process than to use an X-ray angiographicprocess. According to the intravascular echoing process, a catheterhaving an ultrasonic probe on its end is inserted into a blood vessel.The ultrasonic probe transmits an ultrasonic wave whilecircumferentially scanning a location to be evaluated (radial scanning),and receives a reflected wave to produce a received signal. Theamplitude of the received signal is modulated in luminance to produce atomographic image of the blood vessel at the evaluated location.

In actual clinic sites, the property and state of a plaque in the bloodvessel are analyzed based on the produced tomographic image to regard aregion of high luminance as a fibrous component of the plaque and alsoto regard a region of low luminance as a lipid component of the plaque.For example, a plaque containing 80% or more of a region of highluminance is regarded as a hard plaque, and a plaque containing 80% ormore of a region of low luminance is regarded as a soft plaque. In thismanner, a pseudo evaluation of the property and state of plaques ismade.

However, the direct relationship between luminance and plaque state isnot strong enough. It is difficult to analyze the property and state ofplaques accurately in detail according to the analyzing process basedsolely on luminance. There has been a demand for a technique to directlyanalyze the property and state of plaques by determining dynamiccharacteristics of plaques. To meet such a demand, there have beenproposed various techniques to determine dynamic characteristics ofplaques, such as a distortion value and a modulus of elasticity, usingultrasonic energy

Japanese Patent Laid-open No. 2000-229078 discloses a technique fortracking the position of a location to be evaluated in a blood vessel,calculating the modulus of elasticity of the wall of the blood vessel,and evaluating the property and state of a plaque in the blood vessel.

Japanese Patent Laid-open No. Hei 8-10260 discloses a technique forallocating partial regions of two images obtained at respective twotimes, determining a complex conjugate product, determining adisplacement from the gradient of the phase of the complex conjugateproduct, and evaluating the rigidity of a tissue in an examined sample.

Japanese Patent Laid-open No. Hei 5-317313 reveals a technique forcalculating the absolute modulus of elasticity at a location to beevaluated in a blood vessel based on a change in the distance betweentwo particular points and the blood pressure in the blood vessel. Thisdocument also discloses a technique for displaying a tomographic imageof a blood vessel with a different hue added depending on the differencebetween the moduli of elasticity that are determined at respectivelocations to be evaluated in the blood vessel.

Japanese Patent No. 3182479 reveals a technique for determining adisplaying a ratio of moduli of elasticity (indirect modulus ofelasticity) which represent respective elastic levels at an observedpoint and a reference point, rather than calculating an absolute modulusof elasticity.

The above conventional techniques calculate a distortion value and/or amodulus of elasticity to evaluate the hardness of a location to beevaluated in a blood vessel. Actually, the distortion values of a bloodvessel wall and a plaque change periodically because the cross-sectionalarea of the lumen of the blood vessel changes periodically due to bloodpressure changes caused by cardiac beats. Specifically, when the bloodvessel is contracted and expanded, the distortion values change greatly,and when the blood vessel switches from a contracted state to anexpanded state and also from an expanded state to a contracted state,the distortion values do not change significantly.

The blood vessel wall and the plaque are essentially elastic bodies thatare dynamically nonlinear. Stresses in the blood vessel wall and thedistortion values are nonlinearly related to each other. Because of thenonlinearity, the moduli of elasticity of the blood vessel wall and theplaque change with time depending on the contracted and expanded stagesof the blood vessel. In addition, since the blood vessel wall and theplaque are viscous, they need to be treated as viscoelastic bodies.

The conventional techniques referred to above do not analyze the stateof a location to be evaluated based on a time-dependent change of achronological sequence of distortion values and moduli of elasticity.Therefore, it has been impossible with the conventional techniques toanalyze the state of a location to be evaluated, i.e., the property andstate of a plaque, in view of the dynamic nonlinearity andviscoelasticity of the blood vessel wall and the plaque.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an ultrasonicdiagnostic apparatus and method for analyzing the state of a location tobe evaluated in view of the nonlinearity and viscoelasticity of thelocation, based on time-series data of instantaneous distortion valuesat a plurality of times.

Another object of the present invention is to provide an ultrasonicdiagnostic apparatus and method for analyzing the state of a location tobe evaluated in view of the nonlinearity and viscoelasticity of thelocation, based on time-series data of instantaneous moduli ofelasticity at a plurality of times.

According to an aspect of the invention, an ultrasonic diagnosticapparatus has an insert to be inserted into a blood vessel, anultrasonic transducer mounted on the insert for transmitting anultrasonic wave in the blood vessel, receiving a reflection of theultrasonic wave, and producing a received signal representative of thereceived reflection, a distortion value calculator which calculates aninstantaneous distortion value at a location to be evaluated in theblood vessel based on the received signal, an analyzer which analyzes astate of the location to be evaluated using time-series data ofinstantaneous distortion values which are calculated at a plurality oftimes in a chronological sequence by the distortion value calculator,and a display which displays an analyzed result produced by theanalyzer.

According to another aspect of the invention, an ultrasonic diagnosticapparatus has an insert to be inserted into a blood vessel, anultrasonic transducer mounted on the insert for transmitting anultrasonic wave in the blood vessel, receiving a reflection of theultrasonic wave, and producing a received signal representative of thereceived reflection, a modulus-of-elasticity calculator which calculatesan instantaneous modulus of elasticity at a location to be evaluated inthe blood vessel based on the received signal, an analyzer whichanalyzes a state of the location to be evaluated using time-series dataof instantaneous moduli of elasticity which are calculated at aplurality of times in a chronological sequence by themodulus-of-elasticity calculator, and a display which displays ananalyzed result produced by the analyzer.

According to still another aspect of the invention, an ultrasonicdiagnostic method has a step of transmitting an ultrasonic wave in ablood vessel, receiving a reflection of the ultrasonic wave, andproducing a received signal representative of the received reflection, astep of calculating an instantaneous distortion value at a location tobe evaluated in the blood vessel based on the received signal, a step ofanalyzing a state of the location to be evaluated using time-series dataof instantaneous distortion values which are calculated at a pluralityof times in a chronological sequence, and a step of displaying ananalyzed result produced.

According to a further aspect of the invention, an ultrasonic diagnosticmethod has a step of transmitting an ultrasonic wave in a blood vessel,receiving a reflection of the ultrasonic wave, and producing a receivedsignal representative of the received reflection, a step of calculatingan instantaneous modulus of elasticity at a location to be evaluated inthe blood vessel based on the received signal, a step of analyzing astate of the location to be evaluated using time-series data ofinstantaneous moduli of elasticity which are calculated at a pluralityof times in a chronological sequence, and a step of displaying ananalyzed result produced.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an ultrasonic diagnostic system accordingto a first embodiment of the present invention;

FIG. 2 is a diagram showing a concept of a processing sequence of theultrasonic diagnostic system shown in FIG. 1;

FIG. 3 is a flowchart of the processing sequence of the ultrasonicdiagnostic system shown in FIG. 1;

FIG. 4 is a diagram showing the principle of a stabilizing process basedon synchronous addition, which is applicable in step S103 of theprocessing sequence shown in FIG. 3;

FIG. 5 is a diagram showing an example of time-series distortion datacalculated in step S103 of the processing sequence shown in FIG. 3;

FIG. 6 is a diagram showing divided frequency components of thetime-series distortion data analyzed in step S104 of the processingsequence shown in FIG. 3;

FIG. 7 is a diagram showing a phase lag of the time-series distortiondata analyzed in step S105 of the processing sequence shown in FIG. 3;

FIG. 8 is a block diagram of an ultrasonic diagnostic system accordingto a second embodiment of the present invention;

FIG. 9 is a diagram showing a processing sequence of the ultrasonicdiagnostic system shown in FIG. 8; and

FIG. 10 is a flowchart of the processing sequence of the ultrasonicdiagnostic system shown in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1st Embodiment

FIG. 1 shows in block form an ultrasonic diagnostic system according toa first embodiment of the present invention.

As shown in FIG. 1, the ultrasonic diagnostic system according to thefirst embodiment generally comprises an ultrasonic catheter 100, acontroller 200, and a display 300.

The ultrasonic catheter 100 comprises a catheter body (insert) 110 thatcan be inserted into a blood vessel and an ultrasonic probe 120 mountedin the catheter body 110. The ultrasonic probe 120 is disposed in alumen in the catheter body 110, and acts as an ultrasonictransmitter/receiver for transmitting and receiving an ultrasonic wavein the blood vessel. Specifically, the ultrasonic probe 120 is anacoustic transducer and alternately repeats the transmission andreception of ultrasonic waves, so that the single ultrasonic probe 120can transmit and receive ultrasonic waves. The ultrasonic probe 120 mayalternatively comprise an ultrasonic transmitting part and an ultrasonicreceiving part as separate elements.

With the ultrasonic catheter 100 inserted in a location to be evaluatedin the blood vessel, the ultrasonic probe 120 transmits an ultrasonicwave while making radial scanning electronically or mechanically, i.e.,rotating circumferentially, and converts an ultrasonic wave reflectedfrom the blood vessel wall into an electric signal, i.e., a receivedsignal (echo signal). Since the ultrasonic probe 120 receives thereflected ultrasonic wave while making radial scanning, the ultrasonicprobe 120 can produce data (two-dimensional data) of a tomographic imageof the blood vessel perpendicularly across the longitudinal axis of theblood vessel. The ultrasonic diagnostic system according to the firstembodiment may also move the ultrasonic probe 120 for scanning along thelongitudinal axis of the blood vessel, in addition to the radialscanning, for producing three-dimensional volume data representative ofa three-dimensional shape of the blood vessel. The ultrasonic catheter100 itself has a structure which is identical to the structure ofconventional ultrasonic catheters, and will not be described in detailbelow.

When the ultrasonic probe 120 applies an ultrasonic wave to a bloodvessel wall or a plaque, the ultrasonic wave is reflected in the bloodvessel wall or the plaque, and then received by the ultrasonic probe120. Since the blood vessel wall or the plaque is locally deformed anddistorted in response to cardiac beats, the distance between two pointsin an object to be evaluated, i.e., the blood vessel wall or the plaque,changes before and after it is deformed in response to cardiac beats.Therefore, the waveform of the received ultrasonic wave locally movesdue to the change in the distance between the two points in the objectto be evaluated in response to cardiac beats.

The controller 200 will be described below. The controller 200 maycomprise a personal computer or a computer such as a work station. Thecontroller 200 has an interface 210, an instantaneous distortion valuecalculator 220, a time-series distortion value data analyzer 230, and animage data generator 240.

The interface 210 comprises an interface for acquiring a received signalproduced by the ultrasonic catheter 100. The interface 210 according tothe present embodiment captures a received signal, which has beenproduced by radial scanning with ultrasonic waves, over a period of timecomprising at least a plurality of cardiac beats.

The instantaneous distortion value calculator 220 calculatesinstantaneous distortion values at the location to be evaluated in theblood vessel. Specifically, the instantaneous distortion valuecalculator 220 calculates an instantaneous distortion value at eachpoint of time when the blood vessel is contracted and expanded bycardiac beats. The instantaneous distortion value calculator 220calculates an instantaneous distortion value in each sampling periodwhich is shorter than the period of cardiac beats, and performs suchcalculations over a plurality of cardiac beats. In this manner, theinstantaneous distortion value calculator 220 produces time-series dataof instantaneous distortion values (hereinafter referred to as“time-series distortion value data”) at a plurality of times which arecalculated in a chronological sequence. The instantaneous distortionvalue calculator 220 has a tracker 221 for tracking the position of thelocation to be evaluated in the blood vessel. The tracker 221 tracks theposition of the location to be evaluated in the blood vessel accordingto an autocorrelation process or a composite autocorrelation process.Details of the tracker 221 will be described later.

The time-series distortion value data analyzer 230 analyzes the state ofthe location to be evaluated, based on the time-series distortion valuedata produced by the instantaneous distortion value calculator 220. Thetime-series distortion value data analyzer 230 determines whether theplaque in the location to be evaluated is a soft plaque or a hard plaquebased on the dynamic nature of the plaque. Stated otherwise, thetime-series distortion value data analyzer 230 evaluates the proportionof a lipid component of the plaque based on the dynamic nature of theplaque.

That the state of the location to be evaluated is analyzed based on thetime-series distortion value data which includes information about thedynamic nonlinearity and viscoelasticity of the location to beevaluated, such as a blood vessel wall or a plaque, constitutes afeature of the ultrasonic diagnostic system according to the presentembodiment. The time-series distortion value data analyzer 230 shouldpreferably have a first function to divide the time-series distortionvalue data into a plurality of frequency components and analyze thestate of the location to be evaluated based on the intensity of acertain one of the frequency components, and a second function toanalyze the state of the location to be evaluated based on a phase lag(which may be a phase lead) of the time-series distortion value data.However, the time-series distortion value data analyzer 230 may haveeither the first function or the second function. The time-seriesdistortion value data analyzer 230 can also calculate a rate oftime-dependent change of the distortion value to analyze the location tobe evaluated. Details of the functions of the time-series distortionvalue data analyzer 230 will be described later.

The image data generator 240 generates tomographic image data of theblood vessel based on the received signal from the interface 210. If theultrasonic diagnostic system moves the ultrasonic probe 120 for scanningalong the longitudinal axis of the blood vessel, in addition to theradial scanning, then the image data generator 240 generatesthree-dimensional volume data in the blood vessel. The image datagenerator 240 also generates image data representative of the analyzedresults produced by the time-series distortion value data analyzer 230.

The instantaneous distortion value calculator 220, the time-seriesdistortion value data analyzer 230, and the image data generator 240 areimplemented by a CPU which executes a computer program. However, theymay be implemented by a dedicated integrated circuit or circuits.

The display 300 will be described below. The display 300 comprises adisplay device such as a liquid crystal display device or a CRT. Thedisplay 300 displays a tomographic image of the blood vessel generatedbased on the received signal, and also displays the analyzed resultsproduced by the time-series distortion value data analyzer 230.Specifically, the display 300 should preferably display the analyzedresults superimposed on the tomographic image. For example, the display300 may display a different hue added to the tomographic image dependingon the analyzed results with respect to the location to be evaluated,i.e., the blood vessel wall or the plaque.

A processing sequence of the ultrasonic diagnostic system, i.e.,sequence of the ultrasonic diagnostic method according to the presentembodiment will be described below.

FIG. 2 shows a concept of the processing sequence of the ultrasonicdiagnostic system according to the present embodiment. As shown in FIG.2, the processing sequence of the ultrasonic diagnostic system includesdata acquisition (step S11), calculation of instantaneous distortionvalues (step S12), and analysis using time-series distortion value data(step S13) which are successively carried out.

In step S11, the data of a received signal (echo signal) at a pluralityof times (first time through (N+1)th time) in a chronological sequenceover a plurality of cardiac beats is acquired.

In step S12, the instantaneous distortion value calculator 220 acquiresthe data in step S11, and compares the data at kth and (k+1)th times (krepresents an integer in the range from 1 to N), which are twochronologically adjacent times, with each other to calculateinstantaneous distortion values at a plurality of times in achronological sequence. In FIG. 2, the instantaneous distortion valuecalculator 220 calculates a total of N instantaneous distortion values.

In step S13, the time-series distortion value data analyzer 230 analyzesthe state of a location to be evaluated, using the instantaneousdistortion value data representing the instantaneous distortion valuesat the plurality of times which have been calculated in thechronological sequence in step S12. As a result, whether a lesion suchas a plaque or the like exists in the location to be evaluated or not,and also the property and state of the plaque based on the dynamiccharacteristics of the plaque, are determined. Specifically, it isdetermined whether the plaque is a hard plaque or a soft plaque.

FIG. 3 is a flowchart of the processing sequence of the ultrasonicdiagnostic system according to the present embodiment. The processingsequence of the ultrasonic diagnostic system includes a data generatingprocess, a moving point tracking process, a process of calculatinginstantaneous distortion values, an analyzing process based on theintensity of a particular frequency component of time-series distortionvalue data, an analyzing process based on a phase lag of the time-seriesdistortion value data, and a process of displaying analyzed results.Details of these processes will be described below.

(Data Generating Process)

The data generating process is performed in step S101. Specifically, thedata of received signals (echo signals) at a plurality of times in achronological sequence over a plurality of cardiac beats are obtained.Specifically, an ultrasonic wave is applied in radial scanning to ablood vessel, and a reflected wave is received to generate tomographicimage data (two-dimensional data) of the blood vessel at the pluralityof times. In addition to the radial scanning, scanning along thelongitudinal axis of the blood vessel may be performed for an expandedthree-dimensional process. If such an expanded three-dimensional processis carried out, then three-dimensional volume data of the blood vesselare produced.

(Moving Point Tracking Process)

The movement of the point of a location to be evaluated in the lumen ofthe blood vessel is tracked in step S102. Specifically, since the bloodvessel wall is distorted in response to cardiac beats, the point of thelocation to be evaluated is moved. In this step, the movement of thepoint of the location to be evaluated is tracked. Specifically, as shownin FIG. 2, the tracker 221 compares the data at kth and (k+1)th times,which are two chronologically adjacent times, with each other to trackthe moving point. For example, the moving point is tracked based on thedata at the first time and the data at the second time.

The moving point tracking process employs an autocorrelation process ora composite autocorrelation process. Preferably, the compositeautocorrelation process is employed. Details of the compositeautocorrelation process will not be described below. Briefly, however,the composite autocorrelation process has a first stage in which signalsthat are successively acquired in response to cardiac beats aresubjected to orthogonal detection. Then, using a complex envelope signalproduced after the orthogonal detection, a position where thecoefficient of correlation of envelope distributions before and afterthe deformation is highest is roughly estimated locally. In a secondstage of the composite autocorrelation process, the position of thepoint is thoroughly checked from a phase difference at the estimatedposition where the correlation of the envelope distributions is highest.

Generally, when the ultrasonic catheter 100 is positionally shifted,ultrasonic scanning lines applied before and after the deformation crosseach other, tending to cause an error in the detection of the phasedifference. According to the composite autocorrelation process, prior tothe detection of a phase difference, the position of the moving point isroughly estimated using a coefficient of correlation of envelopedistributions which is less susceptible to the positional shift of theultrasonic catheter 100, so that any adverse effect due to the error inthe detection of the phase difference can be minimized. The coefficientof correlation of envelope distributions corresponds to a quantityrepresentative of the degree of agreement between local waveforms beforeand after the deformation.

When the ultrasonic catheter 100 is inserted into a coronary artery,since it is directly affected by the movement of the heart, the relativeposition of the ultrasonic catheter 100 with respect to the blood vesselmay be shifted along the longitudinal axis of the blood vessel duringmeasurement over a long period of time. The composite autocorrelationprocess, however, can be expanded to a three-dimensional processcomprising radial, circumferential, and longitudinal directions. If theultrasonic diagnostic system 100 performs scanning along thelongitudinal axis of the blood vessel in addition to radial scanning,then the composite autocorrelation process allows the ultrasonicdiagnostic system 100 to track the moving point not only in the radialand circumferential direction, but also in the longitudinal direction.

According to the above moving point tracking process, a component u_(r)^((k))=u_(r) ^((k))(r, θ, l; kΔt) in the radial direction r of adisplacement between the kth and (k+1)th times, a componentuθ^((k))=uθ^((k))(r, θ, l; kΔt) in the circumferential direction θ ofthe displacement, and a component u_(l) ^((k))=u_(l) ^((k))(r, θ, l;kΔt) in the longitudinal direction l of the displacement are determined.In the above equations, Δt represents a sampling period, r, θ, lrepresent coordinate values in the three-dimensional volume data at thekth time.

If a spatial point of the location to be evaluated which is to benoticed at the start of the processing sequence is represented by p₀(p_(r) ⁽⁰⁾=r₀, pθ⁽⁰⁾=θ⁽⁰⁾, P_(l) ⁽⁰⁾=l₀), then the position to whichthis spatial point moves after the time kΔt (i.e., kth time) isexpressed by the following equations (1a) through (1c): $\begin{matrix}{p_{r}^{(k)} = {p_{r}^{(0)} + {\sum\limits_{i = 0}^{k - 1}{u_{r}^{(i)}\left( {p_{r}^{(i)},p_{\theta}^{(i)},p_{l}^{(i)}} \right)}}}} & \left( {1a} \right) \\{p_{\theta}^{(k)} = {p_{\theta}^{(0)} + {\sum\limits_{i = 0}^{k - 1}{u_{\theta}^{(i)}\left( {p_{r}^{(i)},p_{\theta}^{(i)},p_{l}^{(i)}} \right)}}}} & \left( {1b} \right) \\{p_{l}^{(k)} = {p_{l}^{(0)} + {\sum\limits_{i = 0}^{k - 1}{u_{l}^{(i)}\left( {p_{r}^{(i)},p_{\theta}^{(i)},p_{l}^{(i)}} \right)}}}} & \left( {1c} \right)\end{matrix}$

After the moving point is tracked as described above, then control goesto the following process. (Process of calculating instantaneousdistortion values)

Then, instantaneous distortion values at a plurality of times in achronological sequence are calculated in step S103. As a result,time-series distortion value data of instantaneous distortion values ata plurality of times in a chronological sequence are obtained.

Specifically, the instantaneous distortion value calculator 220calculates a distortion value ε from a displacement, i.e., (u_(r)^((k)), uθ^((k)), u_(l) ^((k))), that is determined by the moving pointtracking process between the data at the kth time and the data at the(k+1)th time, which are two chronologically adjacent times. Morespecifically, the distortion value ε is obtained by differentiating thecomponents of the displacement with respect to r, θ, l, respectively,according to the following equations (2a) through (2c): $\begin{matrix}{{ɛ_{r}^{(k)}\left( {r,\theta,l} \right)} = \frac{\partial{u_{r}^{(k)}\left( {r,\theta,l} \right)}}{\partial r}} & \left( {2a} \right) \\{{ɛ_{\theta}^{(k)}\left( {r,\theta,l} \right)} = \frac{\partial{u_{\theta}^{(k)}\left( {r,\theta,l} \right)}}{\partial\theta}} & \left( {2b} \right) \\{{ɛ_{l}^{(k)}\left( {r,\theta,l} \right)} = \frac{\partial{u_{l}^{(k)}\left( {r,\theta,l} \right)}}{\partial l}} & \left( {2c} \right)\end{matrix}$

Therefore, the distortion value after t=tΔk of the spatial point (r₀,θ₀, l₀) of the location to be evaluated which is to be noticed at thestart of the processing sequence is calculated according to the equation(3) shown below, using the equations (1a) through (1c) and the equations(2a) through (2c). Though only a component in the radial direction ofthe distortion value is indicated, components in the circumferential andlongitudinal direction can similarly be calculated. $\begin{matrix}\begin{matrix}{{{\overset{\sim}{ɛ}}_{r}\left( {r_{0},\theta_{0},{l_{0};t}} \right)} = {{\overset{\sim}{ɛ}}_{r}\left( {r_{0},\theta_{0},{l_{0};{k\quad\Delta\quad t}}} \right)}} \\{= {{\overset{\sim}{ɛ}}_{r}\left( {r_{0},\theta_{0},l_{0}} \right)}} \\{= {ɛ_{r}^{(k)}\left( {p_{r}^{(k)},p_{\theta}^{(k)},p_{l}^{(k)}} \right)}}\end{matrix} & (3)\end{matrix}$

According to the equation (3), instantaneous distortion values at aplurality of times t (tΔk) are calculated. As a result, time-seriesdistortion value data of instantaneous distortion values at a pluralityof times in a chronological sequence are obtained.

The time-series distortion value data represented by the equation (3)contain noise. Therefore, the instantaneous distortion value calculator220 may further have a function to reduce the noise contained in thetime-series distortion value data. For example, the instantaneousdistortion value calculator 220 may reduce the noise contained in thetime-series distortion value data according to a spatial local smoothingprocess, a bandpass filtering process, or a stabilizing process based onsynchronous addition, which will be described below.

According to the spatial local smoothing process, ROI (regions ofinterest) which are local spatial regions are set up and a smoothingprocess is performed, as indicated by the equation (4) shown below. Inthe equation (4), a three-dimensional spatial ROI is represented by asymbol V. $\begin{matrix}{{{\overset{\_}{ɛ}}_{r}\left( {r,\theta,{l;t}} \right)} = {\frac{1}{V}\quad{\int_{V}^{\quad}{{{\overset{\sim}{ɛ}}_{r}\left( {r^{\prime},\theta^{\prime},{l^{\prime};t}} \right)}{\mathbb{d}V}}}}} & (4)\end{matrix}$

According to the bandpass filtering process, the bandpass filteringprocess which passes only certain frequencies is applied to thetime-series distortion value data which has been subjected to thespatial local smoothing process indicated by the equation (4), accordingto the equation (5) shown below. Specifically, since the time-seriesdistortion value data change periodically in response to cardiac beats,other components than the frequency components corresponding to thecardiac beats are highly likely to be noise. Therefore, by applying abandpass filter set up across a frequency (1 Hz) corresponding to thecardiac beats to the time-series distortion value data, noise can beremoved from the time-series distortion value data, thus extractingstable time-series distortion value data.{circumflex over (ε)}_(r)(r,θ,l;t)=BPF{{overscore (ε)}(r,θ,l;t)}  (5)

A stabilizing process based on synchronous addition may be carried outinstead of the bandpass filtering process. According to the stabilizingprocess based on synchronous addition, based on the fact that thetime-series distortion value data change periodically in response tocardiac beats, values in corresponding time regions are added over aplurality of periods, and averaged. FIG. 4 illustrates the principles ofthe stabilizing process based on synchronous addition.

As shown in FIG. 4, if the period of the time-series distortion valuedata is represented by T, then a region having a width tw with a time tat its center, and regions having respective widths tw with a time t+T,a time t+2T, a time t+3T, . . . , a time t+(N−1)T at their centerscorrespond to each other. The stabilizing process based on synchronousaddition is a process for averaging the values of these correspondingregions. Stated otherwise, the stabilizing process based on synchronousaddition is a chronological local averaging process according to thefollowing equation (6): $\begin{matrix}{{{\hat{ɛ}}_{r}\left( {r,\theta,{l;t}} \right)} = {\frac{1}{N \cdot t_{w}}{\sum\limits_{i = 0}^{N - 1}{\int_{\frac{- t_{w}}{2}}^{\frac{t_{w}}{2}}{{{\overset{\_}{ɛ}}_{r}\left( {r,\theta,{l;{t + {iT} + t^{\prime}}}} \right)}{\mathbb{d}t^{\prime}}}}}}} & (6)\end{matrix}$

As described above, in step S103 shown in FIG. 3, time-series distortionvalue data of distortion values at a plurality of times are calculatedbased on a change in the distance between two local points. According tothe present embodiment, not only the spatial local smoothing process,but also the bandpass filtering process and the stabilizing processbased on synchronous addition, are carried out to produce time-seriesdistortion value data with reduced noise.

An example of the time-series distortion value data is shown in FIG. 5.FIG. 5 shows the time-series distortion value data which are obtainedboth when the location to be evaluated is a soft plaque with a highlipid content and when the location to be evaluated is a hard plaquewith a high fibrous content, a high lime content, and a low lipidcontent.

Then, the state of the location to be evaluated, or preferably the stateof the plaque, is analyzed based on the time-series distortion valuedata of instantaneous distortion values at a plurality of times whichare calculated in a chronological sequence by the instantaneousdistortion value calculator 220. The analysis includes an analysis basedon the intensity of a particular frequency of the time-series distortionvalue data and an analysis based on a phase lag of the time-seriesdistortion value data.

(Analysis Based on the Intensity of a Frequency)

In step S104 shown in FIG. 3, an analysis based on the intensity of aparticular frequency of the time-series distortion value data is carriedout. Specifically, the time-series distortion value data analyzer 230divides the time-series distortion value data into a plurality offrequency components, and analyzes the state of the location to beevaluated based on the intensity of a particular one of the frequencycomponents.

Specifically, a Fourier transform represented by the equation (7) shownbelow is applied to the time-series distortion value data determined atthe spatial points (r, θ, l) of the location to be evaluated, therebydividing the time-series distortion value data into a plurality offrequency components. Actually, the same process is performed on thetime-series distortion value data determined at all the spatial pointsof the location to be evaluated.E _(r)(r,θ,l;w)=∫_(∞) ^(∞) ê _(r)(r,θ,l;t)e ^(−jωt) dt   (7)

FIG. 6 shows the divided frequency components. In FIG. 6, the horizontalaxis represents the frequency ω and the vertical axis the intensity P.The frequency ωc represents the frequency of cardiac beats, and is about1 Hz. FIG. 6 shows the divided frequency components both when thelocation to be evaluated is a hard plaque and when the location to beevaluated is a soft plaque.

As shown in FIG. 6, the frequency intensity is maximum in the vicinityof the cardiac beat frequency of ωc irrespective of whether the locationto be evaluated is a hard plaque or a soft plaque. The frequencyintensity is higher when the location to be evaluated is a soft plaquethan when the location to be evaluated is a hard plaque, and thedifference between these frequency intensities is noticeable in thevicinity of the frequency of ωc.

Then, the time-series distortion value data analyzer 230 calculates theintensity of the frequency components in the vicinity of the cardiacbeat frequency of ωc for each spatial point. As a result, it is possibleto evaluate the property and state of the plaque irrespective of thestage of contraction and expansion of the blood vessel due to cardiacbeats. Specifically, as indicated by the equation (8) shown below, therange of a width ωw with the cardiac beat frequency of ωc at its centeris set up, and the frequency characteristics determined according to theequation (7) is integrated in the above range to calculate the intensityin the vicinity of the cardiac beat frequency of ωc. Specifically, theintensity in the vicinity of the cardiac beat frequency of ωc iscalculated at a plurality of spatial points. $\begin{matrix}{{P_{r}\left( {r,\theta,l} \right)} = {\int_{\omega_{c} - \frac{\omega_{w}}{2}}^{\omega_{c} + \frac{\omega_{w}}{2}}{{{E_{r}\left( {r,\theta,{l;\omega}} \right)}}{\mathbb{d}\omega}}}} & (8)\end{matrix}$

It is preferable to regard a particular position (e.g., a position nearthe outer membrane of the blood vessel) as a reference point andstandardize the intensity at each position based on the reference point.Using standardized values, it is possible to evaluate the hardness ofthe plaque stably in a semi-quantitative manner.

It is also possible to make an evaluation in view of a change in thetime-series frequency intensity by dividing the time-series distortionvalue data calculated over an interval which is at least twice theperiod of cardiac beats into a plurality of frequency components using aFourier transform, and evaluating the hardness of the plaque based onthe frequency intensity in the vicinity of ωc.

Though the component in the radial direction r has been described above,components in the circumferential direction θ and the longitudinaldirection l can also be evaluated by dividing the time-series distortionvalue data into a plurality of frequency components and using thefrequency intensity in the vicinity of ωc.

A process of analyzing the state of the location to be evaluated, i.e.,the state of the plaque, based on the phase lag of the time-seriesdistortion value data will be described below.

(Analysis Based on Phase Lag of Time-Series Distortion Value Data)

According to the present embodiment, the analysis based on the intensityof a particular frequency of the time-series distortion value data (stepS104 in FIG. 3) is followed by an analysis based on the phase lag of thetime-series distortion value data (step S105). Specifically, thetime-series distortion value data analyzer 230 compares the time-seriesdistortion value data obtained at each spatial point and the time-seriesdistortion value data obtained at a reference point (e.g., a positionnear the outer membrane of the blood vessel) with each other, calculatesa phase lag between the time-series distortion value data, and analyzesthe state of the location to be evaluated based on the magnitude of thephase lag.

More specifically, based on the fact that the time-series distortionvalue data determined at each spatial point (r, θ, l) of the location tobe evaluated has ωc as the carrier angular frequency (cardiac angularfrequency), orthogonal detection is performed on the time-seriesdistortion value data using ωc as a reference frequency. As a result, asindicated by the equation (9) shown below, complex time-seriesdistortion value data represented by an envelope portion νr and aportion relative to a phase φ is calculated.{haeck over (ε)}_(r)(rθ,l;t)=ν_(r)(r,θ,l;t)e ^(jφ(r,θ,l;t))   (9)where ν_(r)(r, θ, l; t) represents an envelope portion and φ_(r)(r, θ,l; t) represents a phase. Using the above equation (9), complextime-series distortion value data at any optional spatial point andcomplex time-series distortion value data at a reference point arecalculated.

Then, a phase lag at any optional spatial point with respect to areference point is determined based on the complex time-seriesdistortion value data at the optional spatial point and the complextime-series distortion value data at the reference point.

FIG. 7 schematically shows a phase lag. Specifically, FIG. 7 showstime-series distortion value data at the reference point and time-seriesdistortion value data at the location to be evaluated. In FIG. 7, thephase of the time-series distortion value data at the location to beevaluated lags behind the phase of time-series distortion value data atthe reference point. The phase lag reflects viscoelasticity at thelocation to be evaluated. Specifically, if the plaque at the location tobe evaluated is a soft plaque, then the phase lag is large, and if theplaque at the location to be evaluated is a hard plaque, then the phaselag is small. The reasons are that a hard plaque (stable plaque) whichcontains a lot of relatively hard fibrous components moves substantiallyin synchronism with the movement of the blood vessel wall depending onthe cardiac beats, and a soft plaque (unstable plaque) which contains alot of lipid components and a very small amount of fibrous componentscauses a response delay with respect to the movement of the blood vesselwall in a direction away from the interface with the blood vessel wall.Consequently, the property and state of the plaque can be analyzed byanalyzing the phase lag. Specifically, a phase lag at any optionalspatial point with respect to a reference point is determined by thefollowing equation (10):Δφ_(r)(r,θ,l)=arg{∫ _(∞) ^(∞){haeck over (ε)}_(r)(r,θ,l){haeck over(ε)}_(r0)*(r ₀ ,θ ₀ , l ₀ ;t)dt}  (10)

That is, the product of a complex conjugate of the complex time-seriesdistortion value data at the reference point and the complex time-seriesdistortion value data at each spatial point of the location to beevaluated is determined and integrated with respect to time, and anargument of the result is produced to obtain a phase lag at any optionalspatial point with respect to a reference point. In the equation (10),the time t is used as an integration variable, and the product isintegrated with respect to time in order to taken stability into accountand evaluate the property and state of the plaque without depending onthe stages of contraction and expansion of the blood vessel due tocardiac beats.

Though the component in the radial direction r has been described above,a phase lag of the time-series distortion value data can also bedetermined with respect to components in the circumferential direction θand the longitudinal direction l, and can similarly be evaluated.

As described above, the property and state of the location to beevaluated can be analyzed based on the intensity of a particularfrequency of the time-series distortion value data (step S104) and/orthe phase lag of the time-series distortion value data (step S105).(Display of analyzed results)

Then, the analyzed results are displayed on the display 300 (step S106in FIG. 3). For example, a tomographic image along a plane perpendicularto the longitudinal axis of the blood vessel is displayed, and theanalyzed results of step S104 and/or step S105 are displayed over thetomographic image.

Specifically, a hue is added to each position on the tomographic imagebased on the intensity of a particular frequency of the time-seriesdistortion value data at each spatial point (step S104) and/or the phaselag of the time-series distortion value data (step S105). For example, aplurality of thresholds are set up, and the standard value of theintensity determined in step S104 is compared with those thresholds.Depending on the results of comparison with the thresholds, added huescan be changed. More specifically, a first hue can be applied to aregion where the standard value of the intensity determined in step S104is smaller than a first threshold, a second hue can be applied to aregion where the standard value of the intensity is equal to or greaterthan the first threshold and smaller than a second threshold, and athird hue can be applied to a region where the standard value of theintensity is equal to or greater than the second threshold. Similarly,the value of the phase lag of the time-series distortion value datadetermined in step S105 can be compared with a plurality of thresholds,and added hues can be changed depending on the results of comparisonwith the thresholds.

According to the composite autocorrelation process used to track themoving point in step S102, a correlation coefficient is calculatedsimultaneously with a phase. The correlation coefficient exhibits a highvalue if the distortion value is small and the moving point istranslated on the same plane.

A hard region (e.g., a hard plaque) has a tendency to make a motionclose to a translation when deformed. Therefore, the value of thecorrelation coefficient of the hard region is higher than that of a softregion. Therefore, the value of the correlation coefficient itselfserves to determine the property and state of the location to beevaluated. A distribution of correlation coefficients is advantageous inthat it can stably be calculated. Therefore, when the display 300displays the value of the correlation coefficient as well as theanalyzed results, it can provide important information for the operatorto determine the state of the location to be evaluated.

The value of the correlation coefficient is low in a region which movesfast. For example, a blood stream in the lumen of a blood vessel movesfaster than the wall of the blood vessel. As a consequence, the bloodstream in the lumen of the blood vessel has a lower correlationcoefficient than the wall of the blood vessel.

Therefore, it can be determined whether the location to be evaluated isa blood stream or a blood vessel wall based on the different correlationcoefficients. Specifically, a determined correlation coefficient iscompared with a threshold, and a tomographic image of a region where thecorrelation coefficient is lower than the threshold is displayed with areduced luminance on the display 300. As a result, the luminance of theblood stream is made lower in its entirety than the luminance of theblood vessel wall, so that the boundary between the blood stream and theblood vessel wall can clearly be displayed. Conversely, the luminance ofa region where the correlation coefficient is higher may be increased.The boundary between the bloodstream and the blood vessel wall can thusbe clarified by displaying the tomographic image with the luminance of aparticular region of the tomographic image being changed based on thecorrelation coefficient.

The details of the ultrasonic diagnostic system according to the firstembodiment has been described above. The ultrasonic diagnostic systemaccording to the first embodiment can be modified in various ways. Forexample, in the above description, the analysis based on the intensityof a particular frequency of the time-series distortion value data (stepS104) is followed by the analysis based on the phase lag of thetime-series distortion value data (step S105). The order of theseanalytic steps may be changed. Furthermore, the state of the location tobe evaluated may be analyzed by performing only one of the analysisbased on the intensity of a frequency component (step S104) and theanalysis based on the phase lag of the time-series distortion value data(step S105).

As described above, the ultrasonic diagnostic system according to thepresent embodiment analyzes the state of an object to be evaluated basedon time-series distortion value data of instantaneous distortion valuesobtained at a plurality of times over a period of time which includes atleast a plurality of cardiac beats. The time-series distortion valuedata reflect the nonlinearity and viscoelasticity of a blood vessel walland a plaque at the location to be evaluated. Accordingly, theultrasonic diagnostic system according to the present embodiment cananalyze the state of the location to be evaluated in view of thenonlinearity and viscoelasticity of the blood vessel wall and theplaque.

According to the analysis based on the intensity of a frequencycomponent (step S104), in particular, the hardness of a plaque can beevaluated stably semi-quantitative manner by an index independent of thestages of contraction and expansion of the blood vessel due to cardiacbeats.

According to the analysis based on the phase lag of the time-seriesdistortion value data (step S105), the property and state of a plaquecan be interpreted based on a phase lag which is an index which stronglyreflects the viscoelasticity of the location to be evaluated.

For calculating an instantaneous distortion value at each time, since anoise component is liable to be comprised, not only the spatial localsmoothing process, but also the bandpass filtering process set up acrossa frequency corresponding to cardiac beats, and/or the stabilizingprocess based on synchronous addition, may be applied. Consequently, thenoise component can be reduced and the spatial resolution is preventedfrom being lowered, while at the same time the chronological continuityof data can be maintained.

2nd Embodiment

According to the first embodiment described above, instantaneousdistortion values are calculated, and the state of the location to beevaluated is analyzed based on time-series distortion value data ofinstantaneous distortion values at a plurality of times. A distortionvalue is an index relatively representing the hardness of the locationto be evaluated, and does not directly indicates the hardness of thelocation to be evaluated. A modulus of elasticity (Young's modulus) E isused as an index indicating the hardness of the location to beevaluated.

According to the present embodiment, an instantaneous modulus ofelasticity at a location to be evaluated in a blood vessel iscalculated, and the state of the location to be evaluated is analyzedbased on time-series modulus-of-elasticity data of instantaneousmodulus-of-elasticity values calculated at a plurality of times in achronological sequence. Those parts of the ultrasonic diagnosticapparatus according to the second embodiment which are identical tothose of the ultrasonic diagnostic apparatus according to the firstembodiment are denoted by identical reference numerals.

FIG. 8 shows in block form an ultrasonic diagnostic system according tothe present embodiment. Of the ultrasonic diagnostic system according tothe present embodiment, a display 300 is identical to the display of theaccording to the first embodiment. Therefore, the display 300 will notbe described in detail below.

An ultrasonic catheter 500 according to the present embodiment has acatheter body 510 and a pressure sensor 530 mounted on the surface ofthe catheter body 510. The pressure sensor 530 is disposed at a proximalend of an area where an ultrasonic probe 520 exists and most closely tothe ultrasonic probe 520, for measuring the pressure of the blood (bloodpressure) in a blood vessel.

A controller 400 according to the present embodiment has an interface410, an instantaneous modulus-of-elasticity value calculator 420, atime-series modulus-of-elasticity data analyzer 430, and an image datagenerator 440. The image data generator 440 has the same function as theimage data generator 240 according to the first embodiment.

The interface 410 captures a received ultrasonic signal obtained by theultrasonic probe 520 of the ultrasonic catheter 500 in the same manneras the interface 210 according to the first embodiment, and alsocaptures a signal representing the blood pressure obtained by thepressure sensor 530.

The instantaneous modulus-of-elasticity value calculator 420 calculatesinstantaneous moduli of elasticity at the location to be evaluated inthe blood vessel. Specifically, the instantaneous modulus-of-elasticitycalculator 420 calculates an instantaneous modulus of elasticity at eachpoint of time when the blood vessel is contracted and expanded bycardiac beats. The instantaneous modulus-of-elasticity calculator 420calculates an instantaneous modulus of elasticity in each samplingperiod which is shorter than the period of cardiac beats, and performssuch calculations over a plurality of cardiac beats. In this manner, theinstantaneous modulus-of-elasticity calculator 420 produces time-seriesdata of instantaneous moduli of elasticity (hereinafter referred to as“time-series modulus-of-elasticity data”) at a plurality of times whichare calculated in a chronological sequence. The instantaneousmodulus-of-elasticity calculator 420 has a tracker 421 for tracking theposition of the location to be evaluated in the blood vessel. Thetracker 421 tracks the position of the location to be evaluated in theblood vessel according to an autocorrelation process or a compositeautocorrelation process, as with the tracker 221 according to the firstembodiment.

Generally, the relationship of a modulus of elasticity E, a distortionvalue ε, and a stress σ is given by the equation (11) shown below. Whilethe variables are represented as Scalar quantities, the equation can beexpanded to cover vector quantities.σ=E·ε  (11)

Therefore, an instantaneous modulus of elasticity can be determined bycalculating an instantaneous distortion value and a stress. Theinstantaneous modulus-of-elasticity calculator 420 has a function tocalculate an instantaneous distortion value in the same manner as withthe instantaneous distortion value calculator 220 according to the firstembodiment, and also has a function to calculate a stress. Theinstantaneous modulus-of-elasticity calculator 420 calculates aninstantaneous modulus of elasticity from the instantaneous distortionvalue and the stress.

The time-series modulus-of-elasticity data analyzer 430 analyzes thestate of the location to be evaluated based on the time-seriesmodulus-of-elasticity data obtained by the instantaneousmodulus-of-elasticity calculator 420. The time-seriesmodulus-of-elasticity data analyzer 430 determines whether the plaque inthe location to be evaluated is a soft plaque or a hard plaque based onthe dynamic nature of the plaque. Stated otherwise, the time-seriesmodulus-of-elasticity data analyzer 430 evaluates the proportion of alipid component of the plaque based on the dynamic nature of the plaque.

That the state of the location to be evaluated is analyzed based on thetime-series modulus-of-elasticity data which includes information aboutthe dynamic nonlinearity and viscoelasticity of the location to beevaluated, such as a blood vessel wall or a plaque, constitutes afeature of the ultrasonic diagnostic system according to the presentembodiment. The time-series modulus-of-elasticity data analyzer 430should preferably have a first function to divide the time-seriesmodulus-of-elasticity data into a plurality of frequency components andanalyze the state of the location to be evaluated based on the intensityof a certain one of the frequency components, and a second function toanalyze the state of the location to be evaluated based on a phase lag(which may be a phase lead) of the time-series modulus-of-elasticitydata. However, the time-series modulus-of-elasticity data analyzer 430may have either the first function or the second function. Thetime-series modulus-of-elasticity data analyzer 430 can also calculate arate of time-dependent change of the modulus of elasticity to analyzethe location to be evaluated.

A processing sequence of the ultrasonic diagnostic system, i.e.,sequence of the ultrasonic diagnostic method according to the presentembodiment will be described below.

FIG. 9 shows a concept of the processing sequence of the ultrasonicdiagnostic system according to the present embodiment. As shown in FIG.9, the processing sequence of the ultrasonic diagnostic system includesdata acquisition (step S21), calculation of instantaneous distortionvalues (step S22), calculation of instantaneous moduli of elasticity(step S23), and analysis using time-series modulus-of-elasticity data(step S24) which are successively carried out. The data acquisition(step S21) and the calculation of instantaneous distortion values (stepS22) are the same as those in step S11 and step S12 according to thefirst embodiment shown in FIG. 2.

In step S23, a stress in the blood vessel is calculated, and aninstantaneous modulus of elasticity is calculated from the stress andthe instantaneous distortion value determined in step S12.

In step S24, an analysis is made using time-series modulus-of-elasticitydata. The analysis in step S24 is the same as the analysis in step S13except that the time-series data used for the analysis is nottime-series distortion value data, but time-series modulus-of-elasticitydata.

FIG. 10 is a flowchart of the processing sequence of the ultrasonicdiagnostic system according to the present embodiment. The processingsequence of the ultrasonic diagnostic system includes a data generatingprocess, a moving point tracking process, a process of calculatinginstantaneous moduli of elasticity, an analyzing process based on theintensity of a particular frequency component of time-seriesmodulus-of-elasticity data, an analyzing process based on a phase lag oftime-series distortion value data, and a process of displaying analyzedresults. The data generating process (step S201) and the moving pointtracking process (step S202) are the same as those according to thefirst embodiment, and will not be described in detail below. (Process ofcalculating instantaneous moduli of elasticity)

Instantaneous moduli of elasticity at a plurality of times in achronological sequence are calculated in step S203. As a result,time-series modulus-of-elasticity data of instantaneous moduli ofelasticity at a plurality of times in a chronological sequence areobtained.

Specifically, the instantaneous modulus-of-elasticity calculator 420determines instantaneous moduli of elasticity at a plurality of times.Instantaneous distortion values at a plurality of times can becalculated in the same manner as with the first embodiment, and hencethe process of calculating such instantaneous distortion values will notbe described below. The instantaneous modulus-of-elasticity calculator420 also estimate stresses at the location to be evaluated in the bloodvessel at the respective times. Then, the instantaneousmodulus-of-elasticity calculator 420 calculates instantaneous moduli ofelasticity at a plurality of times based on the instantaneous distortionvalues at the location to be evaluated and the estimated stresses at therespective times.

As tress at the location to be evaluated in the blood vessel wall iscalculated according to a finite element method described below, forexample.

A change in the blood pressure in the lumen of the blood vessel ismeasured by the pressure sensor 530 at the same time that an ultrasonicdiagnosis is made using the ultrasonic catheter 500.

Based on the data obtained in step S201, a region of the blood vesselwall is extracted. The extracted region is divided into a mesh andmodeled. Using the blood pressure change in the lumen of the bloodvessel, a finite element analysis is performed to calculate a stress ineach element of the mesh. The calculated stress is regarded as a localstress in the blood vessel wall. Finally, an instantaneous modulus ofelasticity is calculated using the calculated stress according to theabove equation (11).

In step S203 shown in FIG. 10, as described above, time-seriesmodulus-of-elasticity data of moduli of elasticity at a plurality oftimes are calculated based on a change in the distance between two localpoints.

Then, based on the time-series modulus-of-elasticity data of moduli ofelasticity that are calculated at a plurality of times in achronological sequence by the instantaneous modulus-of-elasticitycalculator 420, the state of the location to be evaluated or preferablythe state of the plaque is analyzed. The analysis includes an analysisbased on the intensity of a particular frequency of the time-seriesmodulus-of-elasticity data and an analysis based on a phase lag of thetime-series modulus-of-elasticity data.

After step S203, the state of the location to be evaluated or preferablythe state of the plaque is analyzed based on the time-seriesmodulus-of-elasticity data of moduli of elasticity that are calculatedat a plurality of times in a chronological sequence by the instantaneousmodulus-of-elasticity calculator 420. The includes an analysis based onthe intensity of a particular frequency of the time-seriesmodulus-of-elasticity data and an analysis based on a phase lag of thetime-series modulus-of-elasticity data.

(Analysis Based on the Intensity of a Frequency)

An analysis based on the intensity of a particular frequency of thetime-series modulus-of-elasticity data is carried out (step S204).Specifically, the time-series modulus-of-elasticity data analyzer 430divides the time-series modulus-of-elasticity data into a plurality offrequency components, and analyzes the state of the location to beevaluated based on the intensity of a particular one of the frequencycomponents. For example, the time-series modulus-of-elasticity dataanalyzer 430 calculates a frequency intensity in the vicinity of thefrequency ωc of cardiac beats. The stronger the frequency intensity andthe greater the modulus of elasticity, the harder the location to beevaluated. That is, a soft plaque has a smaller modulus of elasticitythan a hard plaque. The processing in step S204 is the same as theprocessing in step S104 shown in FIG. 3 except that the time-series datato be processed are time-series modulus-of-elasticity data. Therefore,the processing in step S204 will not be described in detail below.

(Analysis Based on Phase Lag of Time-Series Modulus-of-Elasticity Data)

An analysis based on the phase lag of time-series modulus-of-elasticitydata will be described below.

According to the present embodiment, the analysis based on the intensityof a particular frequency of the time-series modulus-of-elasticity data(step S204) is followed by an analysis based on the phase lag of thetime-series modulus-of-elasticity data (step S205). Specifically, thetime-series modulus-of-elasticity data analyzer 430 compares thetime-series modulus-of-elasticity data obtained at each spatial pointand the time-series modulus-of-elasticity data obtained at a referencepoint (e.g., a position near the outer membrane of the blood vessel)with each other, calculates a phase lag between the time-seriesmodulus-of-elasticity data, and analyzes the state of the location to beevaluated based on the magnitude of the phase lag. In this case, too, ifthe plaque at the location to be evaluated is a soft plaque, then thephase lag is large, and if the plaque at the location to be evaluated isa hard plaque, then the phase lag is small. Consequently, the propertyand state of the plaque can be analyzed by analyzing the phase lag. Theprocessing in step S205 is the same as the processing in step S105 shownin FIG. 3 except that the time-series data to be processed aretime-series modulus-of-elasticity data. Therefore, the processing instep S205 will not be described in detail below.

(Display of Analyzed Results)

Then, the analyzed results are displayed on the display 300 (step S206).The processing in step S206 is the same as the processing in step S106except that the analyzed results produced by the time-seriesmodulus-of-elasticity data analyzer 430 (step S204 and/or step S205),rather than the analyzed results produced by the time-series distortionvalue data analyzer 230 (step S104 and/or step S105), are displayed overthe tomographic image. Therefore, the processing in step S206 will notbe described in detail below.

In the above description, the instantaneous modulus-of-elasticitycalculator 420 calculates the value of an absolute modulus of elasticitybased on the value of the blood pressure in the lumen of the bloodvessel. The present embodiment, however, is not limited to such aprocess. Rather, the instantaneous modulus-of-elasticity calculator 420may calculate a relative modulus of elasticity using a given referenceposition in the blood vessel as a point of reference (i.e., a modulus ofelasticity at the time the distortion value in the entire blood vesselwall is normalized by the distortion value in a certain region).

In the above description, the pressure sensor 530 mounted on theultrasonic catheter 500 is used to obtain the value of the bloodpressure in the lumen of the blood vessel. However, the value of anabsolute modulus of elasticity may be calculated by connecting anordinary electronic tonometer having an arm band to the interface 410.It is preferable to use the pressure sensor 530 mounted on theultrasonic catheter 500 as it can obtain a blood pressure at the samelocation as the region which is ultrasonically diagnosed.

If a calcified area is present on the surface of the lumen of the bloodvessel, then a region of the blood vessel wall cannot be extractedbecause of a shortage of sufficient echo strength behind the calcifiedarea. For this reason, an absolute modulus of elasticity may becalculated for a blood vessel where a region of the blood vessel wallcan be extracted, and a relative modulus of elasticity may be calculatedfor a blood vessel where a region of the blood vessel wall cannot beextracted. In this case, though a relative modulus of elasticity isinvolved, the elasticity of the entire blood vessel wall can beevaluated under the same conditions.

In the above embodiment, the analysis based on the intensity of aparticular frequency (step S204) is followed by the analysis based onthe phase lag of the time-series modulus-of-elasticity data (step S205).The order of these analytic steps may be changed. Furthermore, the stateof the location to be evaluated may be analyzed by performing only oneof the analysis based on the intensity of a frequency component (stepS204) and the analysis based on the phase lag of the time-seriesmodulus-of-elasticity data (step S205).

As described above, the ultrasonic diagnostic system according to thepresent embodiment analyzes the state of an object to be evaluated basedon time-series modulus-of-elasticity data of instantaneous moduli ofelasticity obtained at a plurality of times over a period of time whichincludes at least a plurality of cardiac beats. The time-seriesmodulus-of-elasticity data reflect the nonlinearity and viscoelasticityof a blood vessel wall and a plaque at the location to be evaluated.Accordingly, the ultrasonic diagnostic system according to the presentembodiment can analyze the location to be evaluated in view of thenonlinearity and viscoelasticity of the blood vessel wall and theplaque.

The referred embodiments of the present invention have been describedabove. However, the present invention should not be limited by thoseembodiments, but maybe modified in various ways within the scope of thetechnical concept of the invention.

For example, the analysis based on the time-series distortion value datadescribed in the first embodiment and the analysis based on thetime-series modulus-of-elasticity data described in the secondembodiment may be carried out by a single ultrasonic diagnostic system.

The present invention is applicable to any system for analyzing thestate of a location to be evaluated based on time-series data ofinstantaneous distortion values or instantaneous moduli of elasticity(time-series distortion value data or time-series modulus-of-elasticitydata) calculated at a plurality of times in a chronological sequence,and may employ a data processing process different from the dataprocessing process described in the above embodiments. For example, thestate of the location to be evaluated may be analyzed not based onfrequency characteristics or phase characteristics, but by calculating atime-dependent rate of increase or decrease of moduli of elasticity inthe time-series modulus-of-elasticity data in a time domain or atime-dependent rate of increase or decrease of distortions in thetime-series distortion value data. For example, the state of thelocation to be evaluated may be analyzed directly from a rate ofincrease or decrease in the time-series data shown in FIG. 5. In thiscase, since the state of the location to be evaluated is analyzed basedon the time-series data which includes information about the dynamicnonlinearity and viscoelasticity of the location to be evaluated, suchas a blood vessel wall or a plaque, the state of the location to beevaluated can be analyzed highly accurately in view of the effects ofthe dynamic nonlinearity and viscoelasticity.

According to the present invention, as described above, inasmuch as thestate of a location to be evaluated is analyzed based on time-seriesdata of instantaneous distortion values or instantaneous moduli ofelasticity calculated at a plurality of times in a chronologicalsequence, and the analyzed results are displayed, it is possible todetermine the state of the location to be evaluated in view of theeffects of the dynamic nonlinearity and viscoelasticity of a bloodvessel and a plaque at the location to be evaluated. Consequently, theproperty and state of the blood vessel wall can be analyzed moreintuitively and accurately, allowing operators to make a more objectivediagnosis without significant differences. As a result, the operatorscan select a uniform, appropriate therapeutic process.

In particular, because the instantaneous distortion values orinstantaneous moduli of elasticity are calculated at a plurality oftimes during a period of plural cardiac beats, it is possible todetermine the state of the location to be evaluated in view of all thestates comprising the stages of contraction and expansion of the bloodvessel. The stabilizing process based on synchronous addition, referredto above, can be applied by calculating the instantaneous distortionvalues or instantaneous moduli of elasticity at a plurality of timesduring a period of plural cardiac beats. Consequently, the noisecomponent can be reduced and the spatial resolution is prevented frombeing lowered, while at the same time the chronological continuity ofdata can be maintained.

In particular, since the time-series distortion value data are dividedinto a plurality of frequency components and the state of the locationto be evaluated is analyzed based on the intensity of a certain one ofthe frequency components, different properties and states of plaques canbe distinguished with high sensitivity in view of the effects of thedynamic nonlinearity and viscoelasticity of the plaques and blood vesselwalls by selecting certain frequencies which allow the differencesbetween the properties and states of the plaques and the blood vesselwalls to be easily recognizable.

Because the state of the location to be evaluated is analyzed based onthe phase lag of the time-series data, the properties and states ofplaques and blood vessel walls can be analyzed in view of thedifferences therebetween.

Furthermore, a tomographic image is displayed with a change in theluminance of a region where a correlation coefficient obtained by theautocorrelation process or the composite autocorrelation process is low.Consequently, the analyzed results of the state of the location to beevaluated can be displayed with a clear boundary shown between a bloodstream and a blood vessel wall.

An ultrasonic wave is transmitted while scanning the blood vessel in itscircumferential direction and is also transmitted while scanning theblood vessel in its longitudinal direction, making it possible to trackthe position of the location to be evaluated which moves in thelongitudinal direction of the blood vessel. Therefore, the processingmeans of the present invention is also applicable even if the relativeposition between the ultrasonic wave transmitting means and the bloodvessel tends to be shifted in the longitudinal direction of the bloodvessel. The ultrasonic diagnostic apparatus according to the presentinvention can acquire not only tomographic image data but alsothree-dimensional volume data, and can be expanded in processing to ananalysis of pulse waves.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

1. An ultrasonic diagnostic apparatus comprising: an insert to beinserted into a blood vessel; an ultrasonic transducer mounted on saidinsert for transmitting an ultrasonic wave in the blood vessel,receiving a reflection of the ultrasonic wave, and producing a receivedsignal representative of the received reflection; a distortion valuecalculator which calculates an instantaneous distortion value at alocation to be evaluated in the blood vessel based on said receivedsignal; an analyzer which analyzes a state of the location to beevaluated using time-series data of instantaneous distortion valueswhich are calculated at a plurality of times in a chronological sequenceby said distortion value calculator; and a display which displays ananalyzed result produced by said analyzer.
 2. An ultrasonic diagnosticapparatus according to claim 1, wherein said analyzer divides saidtime-series data into a plurality of frequency components and analyzesthe state of the location to be evaluated based on the intensity of aparticular one of the frequency components.
 3. An ultrasonic diagnosticapparatus according to claim 2, wherein said analyzer judges that theproportion of a lipid component of a plaque formed in said blood vesselis higher as said intensity is higher.
 4. An ultrasonic diagnosticapparatus according to claim 1, wherein said analyzer analyzes the stateof the location to be evaluated based on a phase lag of said time-seriesdata.
 5. An ultrasonic diagnostic apparatus according to claim 4,wherein said analyzer judges that the proportion of a lipid component ofa plaque formed in said blood vessel is higher as said phase lag isgreater.
 6. An ultrasonic diagnostic apparatus according to claim 1,wherein said distortion value calculator tracks the position of thelocation to be evaluated in the blood vessel according to anautocorrelation process or a composite autocorrelation process.
 7. Anultrasonic diagnostic apparatus according to claim 6, wherein saidultrasonic transducer transmits the ultrasonic wave while scanning theblood vessel in a circumferential direction thereof and transmits theultrasonic wave while scanning the blood vessel in a longitudinaldirection thereof.
 8. An ultrasonic diagnostic apparatus according toclaim 7, said distortion value calculator can track the position of thelocation to be evaluated which moves in the longitudinal direction ofthe blood vessel.
 9. An ultrasonic diagnostic apparatus according toclaim 1, wherein said display displays a tomographic image of the bloodvessel based on said received signal and displays the analyzed resultproduced by said analyzer over said tomographic image.
 10. An ultrasonicdiagnostic apparatus according to claim 9, said distortion valuecalculator tracks the position of the location to be evaluated in theblood vessel according to an autocorrelation process or a compositeautocorrelation process.
 11. An ultrasonic diagnostic apparatusaccording to claim 10, said display displays said tomographic image witha change in the luminance of a particular region of said tomographicimage based on a correlation coefficient obtained based on saidautocorrelation process or said composite autocorrelation process. 12.An ultrasonic diagnostic apparatus according to claims 1, wherein saiddistortion value calculator reduces noise comprised in said time-seriesdata.
 13. An ultrasonic diagnostic apparatus comprising: an insert to beinserted into a blood vessel; an ultrasonic transducer mounted on saidinsert for transmitting an ultrasonic wave in the blood vessel,receiving a reflection of the ultrasonic wave, and producing a receivedsignal representative of the received reflection; amodulus-of-elasticity calculator which calculates an instantaneousmodulus of elasticity at a location to be evaluated in the blood vesselbased on said received signal; an analyzer which analyzes a state of thelocation to be evaluated using time-series data of instantaneous moduliof elasticity which are calculated at a plurality of times in achronological sequence by said modulus-of-elasticity calculator; and adisplay which displays an analyzed result produced by said analyzer. 14.An ultrasonic diagnostic apparatus according to claim 13, wherein saidanalyzer divides said time-series data into a plurality of frequencycomponents and analyzes the state of the location to be evaluated basedon the intensity of a particular one of the frequency components.
 15. Anultrasonic diagnostic apparatus according to claim 14, wherein saidanalyzer judges that the proportion of a lipid component of a plaqueformed in said blood vessel is lower as said intensity is higher.
 16. Anultrasonic diagnostic apparatus according to claim 13, wherein saidanalyzer analyzes the state of the location to be evaluated based on aphase lag of said time-series data.
 17. An ultrasonic diagnosticapparatus according to claim 16, wherein said analyzer judges that theproportion of a lipid component of a plaque formed in said blood vesselis higher as said phase lag is greater.
 18. An ultrasonic diagnosticapparatus according to claim 13, wherein said modulus-of-elasticitycalculator tracks the position of the location to be evaluated in theblood vessel according to an autocorrelation process or a compositeautocorrelation process.
 19. An ultrasonic diagnostic apparatusaccording to claim 18, wherein said ultrasonic transducer transmits theultrasonic wave while scanning the blood vessel in a circumferentialdirection thereof and transmits the ultrasonic wave while scanning theblood vessel in a longitudinal direction thereof.
 20. An ultrasonicdiagnostic apparatus according to claim 19, said modulus-of-elasticitycalculator can track the position of the location to be evaluated whichmoves in the longitudinal direction of the blood vessel.
 21. Anultrasonic diagnostic apparatus according to claim 13, wherein saiddisplay displays a tomographic image of the blood vessel based on saidreceived signal and displays the analyzed result produced by saidanalyzer over said tomographic image.
 22. An ultrasonic diagnosticapparatus according to claim 21, said modulus-of-elasticity calculatortracks the position of the location to be evaluated in the blood vesselaccording to an autocorrelation process or a composite autocorrelationprocess.
 23. An ultrasonic diagnostic apparatus according to claim 22,said display displays said tomographic image with a change in theluminance of a particular region of said tomographic image based on acorrelation coefficient obtained based on said autocorrelation processor said composite autocorrelation process.
 24. An ultrasonic diagnosticapparatus according to claims 23, wherein said modulus-of-elasticitycalculator reduces noise comprised in said time-series data.
 25. Anultrasonic diagnostic apparatus according to claim 13, wherein saidmodulus-of-elasticity calculator calculates an absolute modulus ofelasticity based on a blood pressure.
 26. An ultrasonic diagnosticapparatus according to claim 13, wherein said modulus-of-elasticitycalculator calculates a relative modulus of elasticity using a givenreference point in said blood vessel as a point of reference.
 27. Anultrasonic diagnostic method comprising steps of: (1) transmitting anultrasonic wave in a blood vessel, receiving a reflection of theultrasonic wave, and producing a received signal representative of thereceived reflection; (2) calculating an instantaneous distortion valueat a location to be evaluated in the blood vessel based on said receivedsignal; (3) analyzing a state of the location to be evaluated usingtime-series data of instantaneous distortion values which are calculatedat a plurality of times in a chronological sequence; and (4) displayingan analyzed result produced in said step (3).
 28. An ultrasonicdiagnostic method according to claim 27, wherein said step (3) comprisesthe step of dividing said time-series data into a plurality of frequencycomponents and analyzing the state of the location to be evaluated basedon the intensity of a particular one of the frequency components.
 29. Anultrasonic diagnostic method according to claim 28, wherein said step(3) comprises the step of judging that the proportion of a lipidcomponent of a plaque formed in said blood vessel is higher as saidintensity is higher.
 30. An ultrasonic diagnostic method according toclaim 27, wherein said step (3) comprises the step of analyzing thestate of the location to be evaluated based on a phase lag of saidtime-series data.
 31. An ultrasonic diagnostic method according to claim30, wherein said step (3) comprises the step of judging that theproportion of a lipid component of a plaque formed in said blood vesselis higher as said phase lag is greater.
 32. An ultrasonic diagnosticmethod according to claim 27, wherein said step (2) comprises the stepof tracking the position of the location to be evaluated in the bloodvessel according to an autocorrelation process or a compositeautocorrelation process.
 33. An ultrasonic diagnostic method accordingto claim 32, wherein said step (1) comprises the step of transmittingthe ultrasonic wave while scanning the blood vessel in a circumferentialdirection thereof, and transmitting the ultrasonic wave while scanningthe blood vessel in a longitudinal direction thereof.
 34. An ultrasonicdiagnostic method according to claim 33, wherein said tracking stepallows tracking of the position of the location to be evaluated whichmoves in the longitudinal direction of the blood vessel.
 35. Anultrasonic diagnostic method according to claim 27, wherein'said step(4) comprises the step of displaying a tomographic image of the bloodvessel based on said received signal and displaying the analyzed resultover said tomographic image.
 36. An ultrasonic diagnostic methodaccording to claim 35, wherein said step (2) comprises the step oftracking the position of the location to be evaluated in the bloodvessel according to an autocorrelation process or a compositeautocorrelation process.
 37. An ultrasonic diagnostic method accordingto claim 36, wherein said step (4) comprises the step of displaying saidtomographic image with a change in the luminance of a particular regionof said tomographic image based on a correlation coefficient obtainedbased on said autocorrelation process or said composite autocorrelationprocess.
 38. An ultrasonic diagnostic method according to claim 27,wherein said step (2) comprises the step of reducing noise comprised insaid time-series data.
 39. An ultrasonic diagnostic method comprisingsteps of: (1) transmitting an ultrasonic wave in a blood vessel,receiving a reflection of the ultrasonic wave, and producing a receivedsignal representative of the received reflection; (2) calculating aninstantaneous modulus of elasticity at a location to be evaluated in theblood vessel based on said received signal; (3) analyzing a state of thelocation to be evaluated using time-series data of instantaneous moduliof elasticity which are calculated at a plurality of times in achronological sequence; and (4) displaying an analyzed result producedin said step (3).
 40. An ultrasonic diagnostic method according to claim39, wherein said step (3) comprises the step of dividing saidtime-series data into a plurality of frequency components and analyzingthe state of the location to be evaluated based on the intensity of aparticular one of the frequency components.
 41. An ultrasonic diagnosticmethod according to claim 40, wherein said step (3) comprises the stepof judging that the proportion of a lipid component of a plaque formedin said blood vessel is lower as said intensity is higher.
 42. Anultrasonic diagnostic method according to claim 39, wherein said step(3) comprises the step of analyzing the state of the location to beevaluated based on a phase lag of said time-series data.
 43. Anultrasonic diagnostic method according to claim 42, wherein said step(3) comprises the step of judging that the proportion of a lipidcomponent of a plaque formed in said blood vessel is higher as saidphase lag is greater.
 44. An ultrasonic diagnostic method according toclaim 39, wherein said step (2) comprises the step of tracking theposition of the location to be evaluated in the blood vessel accordingto an autocorrelation process or a composite autocorrelation process.45. An ultrasonic diagnostic method according to claim 44, wherein saidstep (1) comprises the step of transmitting the ultrasonic wave whilescanning the blood vessel in a circumferential direction thereof, andtransmitting the ultrasonic wave while scanning the blood vessel in alongitudinal direction thereof.
 46. An ultrasonic diagnostic methodaccording to claim 45, wherein said tracking step allows tracking of theposition of the location to be evaluated which moves in the longitudinaldirection of the blood vessel.
 47. An ultrasonic diagnostic methodaccording to claim 39, wherein said step (4) comprises the step ofdisplaying a tomographic image of the blood vessel based on saidreceived signal and displaying the analyzed result over said tomographicimage.
 48. An ultrasonic diagnostic method according to claim 47,wherein said step (2) comprises the step of tracking the position of thelocation to be evaluated in the blood vessel according to anautocorrelation process or a composite autocorrelation process.
 49. Anultrasonic diagnostic method according to claim 48, wherein said step(4) comprises the step of displaying said tomographic image with achange in the luminance of a particular region of said tomographic imagebased on a correlation coefficient obtained based on saidautocorrelation process or said composite autocorrelation process. 50.An ultrasonic diagnostic method according to claim 39, wherein said step(2) comprises the step of reducing noise comprised in said time-seriesdata.
 51. An ultrasonic diagnostic method according to claim 39, whereinsaid step (2) comprises the step of calculating an absolute modulus ofelasticity based on a blood pressure.
 52. An ultrasonic diagnosticmethod according to claim 39, wherein said step (2) comprises the stepof calculating a relative modulus of elasticity using a given referencepoint in said blood vessel as a point of reference.